1. Field of the Invention
This invention relates to a semiconductor device having a sealing film and a manufacturing method thereof.
2. Description of the Related Art
There has been known a semiconductor device called a chip size package (CSP) described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-22052. The semiconductor device described in this Jpn. Pat. Appln. KOKAI Publication has a structure in which wiring lines are provided on a semiconductor substrate, and columnar electrodes are provided on the upper surface of connection pad portions of the wiring lines, and then a sealing film covers the semiconductor substrate except for these columnar electrodes. In this case, in order to reduce stress caused by the difference between the thermal expansion coefficients of the semiconductor substrate and the sealing film, the sealing film has a three-layer structure which includes, starting from the side of the semiconductor substrate, a lower sealing film or layer having a large amount of silica particles mixed in epoxy resin, an intermediate sealing film or layer having a small amount of silica particles mixed in epoxy resin, and an upper sealing film or layer made of epoxy resin alone.
In a general sealing film forming method that is known, a sealing film formation film made of liquid epoxy resin with or without silica particles therein is formed on the semiconductor substrate by screen printing, and the liquid epoxy resin in the formation film is heated and hardened to form a sealing film.
On the other hand, if the screen printing is carried out in the atmosphere, the liquid resin is pressed on a screen by a squeegee, so that air is caught into the formation film as air bubbles. If such a film is hardened as it is, its strength and moisture resistant properties deteriorate because the air bubbles are scattered in the sealing film. Therefore, it is possible to carry out vacuum deaeration in a vacuum chamber after the screen printing, in order to remove the air bubbles scattered in the formation film.
However, in the conventional semiconductor device described above, the lower sealing layer of the sealing film is formed by the material having a large amount of silica particles mixed in the epoxy resin, so that when the diameter of the silica particle is greater than the distance between the wiring lines, the silica particle may not be contained between the wiring lines and may be placed to extend over the adjacent wiring lines. In such a case, when the formation film is formed by the screen printing, there is a problem that air tends to remain as air bubbles in a gap under the silica particles placed to extend over the adjacent wiring lines, and the air bubbles are not easily removed by the vacuum deaeration and remain as they are between the wiring lines.